You are:

The widest motorway

There are sections of motorway where local widening occurs to accommodate slip roads, and at these points, the motorways could well be five or even six lanes wide. However, the foreshore section of the M2 in Belfast, opened in 1973, has a long section which is dual 5 lanes wide. More recently, opening in December 2005, the M25, between junctions 12 and 14, was widened to dual 5-lanes and the section between junctions 14 and 15 to dual 6-lanes.